Process for preparing coated alkali metal percarbonate, coated alkali metal percarbonate obtainable by this process, its use in detergent compositions, and detergent compositions containing it

ABSTRACT

A process for preparing coated particulate alkali metal percarbonate in which coating is carried out by mixing percarbonate with a solid particulate coating and agent and optionally with an aqueous liquid, so that the amount of water present during mixing does not exceed 12% of the total weight of the mixture. Coated alkali metal percarbonate obtainable by this process and its use as active bleach in detergent compositions is also disclosed.

The present invention is related to a process for preparing coatedalkali metal percarbonate and to the coated alkali metal percarbonateobtainable by this process. It is further related to the use of thiscoated alkali metal percarbonate in detergent compositions and todetergent compositions containing it.

It is known to use alkali metal percarbonate as bleaching compound indetergent powder mixtures. It is further known that alkali metalpercarbonate decomposes quickly during storage in the powdered state.One way to stabilize alkali metal percarbonate is by coating it withboric acid. For instance, the British patent 1 575 792 describes thecoating of sodium percarbonate with a solution of boric acid. Thisprocess however suffers from the following problem. Due to the limitedsolubility of boric acid, particularly at ambient temperature, it isnecessary to use large volumes of coating solution or multiple coatingsteps to obtain a sufficient amount of boric acid on the surface of thepercarbonate particles. A solution to this problem has been proposed inthe international patent application WO 95/15292 wherein a solubilizingamount of a neutral salt is added in the coating solution to enable agreater weight of boric acid to be applied on the surface of thepercarbonate.

The purpose of the present invention is to furnish a new process for thecoating of percarbonate particles comprising a limited amount of processsteps, which does not require the preparation of solutions andevaporation of large volumes of water, which is easy to carry out andwhich enables to use boric acid alone as a coating agent.

To this end, the present invention is related to a process for preparingcoated alkali metal percarbonate in which coating is carried out bymixing alkali metal percarbonate with at least one solid particulatecoating agent and optionally with an aqueous liquid, so that the amountof water present during mixing does not exceed 12% of the total weightof the mixture. The alkali metal percarbonate is preferably sodiumpercarbonate.

One of the essential characteristics of the present invention resides inthe use of a coating agent in the solid form. In other words, when thealkali metal percarbonate is mixed with the coating agent, this coatingagent is introduced into the mixture in the solid form (or powder formor particulate form) instead of in the form of a solution or slurry. Oneof the advantages is that, before this mixing, it is not necessary toprepare, in a preliminary step, a solution or a slurry of the coatingagent. This makes the process economically more interesting.

Another characteristic of the present invention resides in the limitedamount of water present during the coating. This amount of water doesgenerally not exceed 10%, in particular 8% of the total weight of themixture containing the alkali metal percarbonate, the coating agent andwater. This low amount of water is not sufficient to solubilize thesolid coating agent. So, the coating agent particles remain solid whenmixed with the percarbonate particles. It is believed that the coatingtakes place due to adsorption of the solid grains of coating agent onthe wet or wetted surface of percarbonate particles, whereby a coatinglayer is formed which is sufficiently homogenous and uniform to givegood stabilizing properties to the percarbonate. The amount of waterpresent during the coating is generally at least 1.5% of the totalweight of the mixture, in particular at least 3%, and preferably atleast 6%

One of the advantages of the process of the invention is that boric acidalone can be used to coat efficiently percarbonate particles withouthaving to prepare large volumes of solution. Further advantages willbecome clear below.

The aqueous liquid added into the mixture during coating can be purewater. Alternatively, the aqueous liquid can be an effluent recycledfrom another process step. For instance, the mother liquors from thecrystallization of the alkali metal percarbonate can be used as aqueousliquid. In another alternative, a combination of pure water with aneffluent, in particular mother liquor from the crystallization, is used.

The coating agent can be chosen from silicates, sulphates, phosphates,carbonates, borates and boric acids. Good results have been obtainedwith boric acids, phosphates and mixture thereof. The amount ofparticulate coating agent used in the process of the invention isgenerally at least 0.1% of the total weight of the mixture containingthe alkali metal percarbonate, the coating agent and water, inparticular at least 1%, and preferably at least 1.5%. The amount ofparticular coating agent is usually at most 15% of the total weight ofthe mixture, in particular at most 7%, and preferably at most 2.5%.

The coating agent used in the process of the invention preferablycontains boric acid. In this case, it generally contains from 30 to 100%by weight of boric acid. It preferably contains at least 96% by weightof boric acid, in particular at least 98% by weight of boric acid. Acoating agent consisting essentially of boric acid, in particularcontaining 100% by weight of boric acid, gives excellent results. Theboric acid can be chosen from orthoboric acid, metaboric acid andtetraboric acid. Orthoboric acid is preferred.

When boric acid is used in the process of the invention, it is generallyused in such a quantity that the alkali metal percarbonate is coatedwith at least 0.1, in particular at least 0.15% by weight of boroncompared to the total weight of the coated alkali metal percarbonate. Inmost cases, the alkali metal percarbonate is coated with at most 0.5, inparticular at most 0.2% by weight of boron compared to the total weightof the coated alkali metal percarbonate. Coating levels of about 0.3% byweight of boron give good results. When coating levels lower than 0.1%by weight of boron are used, the stabilising effect is too low for itsuse in detergent compositions. When coating levels higher than 0.35% ofboron are used, agglomeration of the coated percarbonate particles intolarge particles can occur.

In the process of the invention, it is advantageous to use a coatingagent powder of a granulometry which is compatible with the granulometryof the percarbonate particles so that, when mixed together, the coatingagent grains can adsorb on the surface of the wet or wetted percarbonateparticles. Preferably, the coating agent has a smaller particle sizethan the alkali metal percarbonate. The smaller the size of the coatingagent particles, the better results are obtained. For instance, when thecoating agent powder grains have a size which is similar to that of thepercarbonate particles, adsorption will not (or at a low extent) takeplace. Generally, the coating agent powder particles have a meandiameter of at most 500 μm, in particular at most 250 μm, for instanceat most 200 μm. The process can also be carried out with coating agentpowder particles of a mean diameter of at most 150 μm. The ratio of themean diameter of the coating agent particles to the mean diameter of theuncoated percarbonate particles is in most cases at least 0.001, inparticular at least 0.01 and preferably at least 0.1. The ratio iscommonly at most 1, in particular at most 0.7 and preferably at most0.3.

The mean diameter of particles is measured according to the proceduredescribed in the ISO standard 3118 using a series of sieves which retainthe particles with a diameter smaller than respectively 75 μm, 150 μm,250 μm, 425 μm, 600 μm, 850 μml, 1000 μm, 1180 μm and 1400 μm.

A mixture of two or more different solid particulate coating agents canbe used in the process of the invention, for instance a mixture of boricacid and phosphate.

The alkali metal percarbonate which is mixed with the coating agent inthe process of the invention can be any known alkali metal percarbonate.

When boric acid is used in the process of the invention, the coating canbe carried out in the absence of other coating agents different fromboric acid.

When boric acid is used in the process of the invention, the coating canbe carried out in the absence of a solubilizing neutral salt.

In the process of the invention, the coating can be carried out at atemperature ranging from ambient temperature up to 60° C. In particular,the temperature ranges from 10 to 30° C. During coating the mixture isgenerally not cooled. It can be advantageous to heat up the aqueousliquid before it is added to the mixture. Heating can be done until atemperature between 45 and 65° C. is reached, and preferably between 50and 60° C. Heating up the aqueous liquid can improve the stability ofthe coated percarbonate.

The duration of the coating is generally at least 10 s, in particular atleast 15 s. The duration is usually at most 15 min, in particular atmost 10 min. Good results are obtained with a duration ranging from 20 sto 5 min.

Mixing of the coating agent powder with the percarbonate can be carriedout in any adequate mixing apparatus such as a plough share mixer. Goodresults are obtained when a plough share mixer of the LODIGE® type isused for the mixing.

In a first alternative of the process of the invention, the coating iscarried out by using dry particles of alkali metal percarbonatecontaining less than 1.5% by weight of water, in particular less than 1%by weight of water, a water content of at most 0.8% by weight being mostpreferred.

The dry particles can, in a first variant, be obtained by a liquidcrystallization process followed by a drying step. In a second variant,the dry particles can be obtained directly from a fluid bed granulationprocess.

According to the first variant, the process can comprise the followingsteps:

-   (1) Preparing an aqueous slurry of alkali metal percarbonate    particles in a crystallizer or crystallizer-classifier. This can be    done by any known liquid crystallization process.-   (2) Separating the alkali metal percarbonate particles from the    aqueous slurry to form a wet cake of alkali metal particles. This    separation can for instance be done in a centrifuge.-   (3) Drying the wet cake of alkali metal percarbonate particles to    form dry alkali metal percarbonate particles containing less than    1.5% by weight of water. This drying operation can for instance be    done in a fluid bed dryer.-   (4) Mixing the dry alkali metal percarbonate particles with the    solid coating agent and with an aqueous liquid so that the amount of    water present during mixing does not exceed 12% of the total weight    of the mixture, to form wet coated alkali metal percarbonate    particles. This step can be carried out by first mixing the dry    alkali metal percarbonate particles with the solid coating agent,    and then adding the aqueous liquid to this mixture. Alternatively,    this step can be carried out by mixing the dry alkali metal    percarbonate particles simultaneously with the aqueous liquid and    the solid coating agent.-   (5) Drying the wet coated alkali metal percarbonate particles to    form dry coated PCS particles containing less than 1.5% by weight of    water, in particular less than 1% by weight of water, a water    content of at most 0.8% by weight being most preferred;-   (6) Collecting the dry coated alkali metal percarbonate particles.

In a second alternative of the process of the invention, the coating iscarried out by using wet particles of alkali metal percarbonate, inparticular particles which have not been dried after being prepared in aliquid crystallization process. The process can then comprise thefollowing steps:

-   (1) As step (1) above;-   (2) As step (2) above;-   (3) mixing the wet cake of alkali metal percarbonate particles with    the solid coating agent, to form a wet cake of coated alkali metal    percarbonate particles. Dry alkali metal percarbonate particles can    optionally be added into the mixture in order to control the water    content of the mixture so that it remains below 12% of the total    weight of the mixture, preferably below 10% or even 8%. This step is    advantageously carried out without adding an aqueous liquid;-   (4) drying the wet cake of coated alkali metal percarbonate    particles to form dry coated alkali metal percarbonate particles;-   (5) collecting the dry coated alkali metal percarbonate particles.

This process is particularly advantageous because there is no dryingstep needed between the crystallization and the coating.

The process of the invention enables to produce thermally stable coatedalkali metal percarbonate which can be used as bleaching compound indetergent formulations.

Consequently, the present invention also concerns the coated alkalimetal percarbonate obtainable by the process described above.

The coated alkali metal percarbonate of the invention generally presentsa surface structure which is substantially in the form of plates whenanalysed with scanning electron microscopy (SEM) at a magnification ofabout 10.000. In particular, the surface structure contains only a fewneedles and is in particular essentially free of needles.

The coated alkali metal percarbonate of the invention has generally amean diameter of at least 500 μm, in particular at least 550 μm. Themean diameter is usually at most 1200 μm, in particular at most 900 μm.

The coated alkali metal percarbonate of the invention presents usuallyan Avox of at least 12%, preferably at least 13.9% by weight. The Avoxis generally at most 14.8%, in particular at most 14.6% by weight. TheAvox is the available oxygen found in the alkali metal percarbonate andindicates the amount of oxygen utilisable in a chemical reaction. It ismeasured by titration with potassium permanganate after dissolution insulfuric acid (see ISO 1917-1982).

The coated alkali metal percarbonate of the invention usually has a bulkdensity of at least 0.8, in particular at least 0.9 g/cm³. It isgenerally at most 1.2, in particular at most 1.1 g/cm³. The bulk densityis measured by recording the mass of sample in a stainless steelcylinder of internal height and diameter of 86.1 mm, after running thesample out of a funnel (upper internal diameter 108 mm, lower internaldiameter 40 mm, height 130 mm) placed 50 mm directly above the receiver.

The coated alkali metal percarbonate of the invention has usually a 90%dissolution time of at least 0.5, in particular at least 0.9 min.Generally. the 90% dissolution time is at most 3, in particular at most2.5 min. The 90% dissolution time is the time taken for conductivity toachieve 90% of its final value after addition of coated alkali metalpercarbonate to water at 15° C. and 2 g/l concentration. The method usedis adapted from ISO 3123-1976 for industrial perborates, the onlydifferences being the stirrer height that is 1 mm from the beaker bottomand a 2 liter beaker (internal diameter 120 mm).

The coated alkali metal percarbonate of the invention usually has anattrition measured according to the ISO standard method 5937-1980 of atleast 0.05, in particular at least 0.1%. The attrition is usually atmost 5, in particular at most 4%.

The coated alkali metal percarbonate of the invention usually presents athermal stability, measured using microcalorimetry at 40° C., of atleast 0.1, in particular at least 0.2 μW/g. It is most preferably atleast 0.3 μW/g. It is generally at most 12, in particular at most 4μW/g. Values of at most 3 μW/g give good results. The measurement ofthermal stability consists of using the heat flow or heat leakageprinciple using a LKB 2277 Thermal Activity Monitor. The heat flowbetween an ampoule containing the coated alkali metal percarbonate and atemperature controlled water bath is measured and compared to an inertreference material consisting of lass balls of a diameter 1-2 mmsupplied by BDH Chemicals Ltd Poole England.

The coated alkali metal percarbonate of the invention can advantageouslybe used as active bleach constituent in detergent compositions.

The present invention therefore concerns also the use of the abovedescribed coated alkali metal percarbonate as active bleach in detergentcompositions.

The present invention also concerns detergent compositions containingthe above described coated alkali metal percarbonate as active bleachconstituent and a builder, either zeolitic or non-zeolitic. Thedetergent compositions can also contain other constitutents such assurfactants, anti-redeposition and soil suspension agents, bleachactivators, optical brightening agents, soil release agents, sudscontrollers, enzymes, fabric softening agents, perfumes, colours andprocessing aids.

The detergent compositions can take any form such as powders, tablets,liquids, etc.

EXAMPLES 1 AND 3 (ACCORDING TO THE INVENTION) AND 2 (COMPARATIVE)

In the example 1, sodium percarbonate (PCS) has been coated according tothe process of the invention (using a boric acid powder). In thecomparative example 2, PCS has been coated with a solution of boricacid. The surface structure of the coated PCS obtained in each examplehas then been analysed by SEM.

The characteristics of the initial uncoated sodium percarbonate are

AVOX 143.9 g/kg Mean particle size 564 μm Bulk Density 1070 g/l

The coating is carried out in two steps. In the first step, 1 kg ofuncoated PCS is mixed in a Lödige mixer M5R (series n° 1099, 5 litrescapacity) under a rotation speed of 200-300 rpm. The coating agents areadded continuously in the upper part of the mixer during the 2 minutesfollowing the starting of the apparatus. The total mixing time is fixedto 5 minutes. The composition and conditions of the mixing step are:

Example 1: Example 2: Example 3: H₃BO₃ powder H₃BO₃ solution Na₂ HPO₄H₃BO₃ or 1.7 1.7 1.8 Na₂HPO₄ (% w/w in total mix) Water (% w/w in 8 8 8total mix) Temperature of Ambient Ambient Ambient mixer (° C.)

In example 1, H₃BO₃ in powder form and water are added in the mixerseparately, in the same time at ambiant temperature on the dry product.In example 2, H₃BO₃ is dissolved in the water at 70° C. and added in themixer on the dry product. In example 3, Na₂ HPO₄ in powder form andwater are added in the mixer separately in the same time at ambianttemperature on the dry product.

In the second step of the process, the wet cake coming from the mixer istransferred in a lab fluid bed dryer (Aeromatic Fielder Strea 1, seriesn° 9799-0710). The conditions of drying step are:

Example 1: Example 2: Example 3: H₃BO₃ powder H₃BO₃ solution Na₂ HPO₄Temperature of air (° C.) 70 70 70 Drying time (min) 30 30 30 Pressure(bar) 4 4 4 Air flow (m³/h) 100 100 100

The product is removed from the dryer after cooling under 40° C. FIG. 1shows the SEM picture (magnification of 10,000) obtained by analysingthe coated PCS of example 1. FIG. 2 shows the same for example 2. TheSEM pictures were taken with a microscope LEO 982. Before SEM analysis,the samples were treated by depositing thereon a 30 nm carbon layer (byevaporation of carbone).

The coated PCS of example 3 presented an available oxygen content of14.2% by weight and a mean diameter of 751 μm.

EXAMPLE 4 (ACCORDING TO THE INVENTION)

The coated sodium percarbonate of example 1 has been incorporated in atablet detergent formulation suitable for dish washing. The detergentcomposition contains a compressed portion and a non-compressed portionwhich is coated with a coating layer. The different portions have thefollowing composition:

-   Compressed portion (20.0 g): tri-sodium citrate dihydrate : 26.4% wt    -   amorphous sodium silicate (SiO₂:Na₂O ratio=1.6-3.2): 26.4% wt    -   amylolytic enzyme: 0.6% wt    -   coated sodium percarbonate : 8.48% wt    -   nonionic surfactant: 1.5% wt    -   tetraacetyl ethylene diamine (TAED): 4.33% wt ethane        1-hydroxy-1,1-diphosphonic acid (HEDP): 0.67% wt    -   diethyltriamine penta (methylene) phosphonate (DETPMP): 0.65% wt    -   paraffin: 0.42% wt    -   benzotriazole (BTA): 0.24% wt    -   polyacrylic acid of average molecular weight    -   approximately 4,500: 3.2% wt    -   anhydrous sodium sulphate: 24.05% wt-   Non-compressed portion (5.0 g): proteolytic enzyme 12.8% wt    -   amylolytic enzyme: 7.2% wt    -   tri-sodium citrate dihydrate: 35.0% wt    -   amorphous sodium silicate (SiO₂:Na₂O ratio=1.6-3.2): 42.0% wt-   Coating layer (1.0 g) starch 15.0% wt    -   C12 dicarboxylic acid: 85.0% wt

EXAMPLE 5 (ACCORDING TO THE INVENTION)

The coated sodium percarbonate of example 1 has been incorporated in atablet detergent formulation suitable for bleaching cloths. Thedetergent tablets have the following composition:

-   Linear alkylbenzene sulphonate: 7.20% wt-   nonionic surfactant: 3.08% wt-   soap: 1.95% wt-   zeolite 4A : 30.38% wt-   polymer (acrylic/maleic): 4.58% wt-   sodium alkaline silicate: 0.53% wt-   sodium carboxymethylcellulose : 0.68% wt-   minor ingredients : 2.03% wt-   moisture: 13.50% wt-   coated sodium percarbonate: 22.75% wt-   TAED:2.25% wt

EXAMPLE 6 (ACCORDING TO THE INVENTION)

The coated sodium percarbonate of example 1 has been incorporated in apowder detergent formulation. The detergent powder has the followingcomposition:

-   Base (8.75 g):sodium linear alkylbenzene sulphonate 16.8% wt    -   nonionic surfactant: 7.5% wt    -   anhydrous zeolite: 44.8% wt    -   acrylic/melaic copolymer: 7.5% wt    -   sodium carbonate: 3.7% wt    -   minor ingredients: 2.9% wt    -   moisture: 16.8% wt    -   coated sodium percarbonate: 1.25 g

EXAMPLE 7 (ACCORDING TO THE INVENTION)

The coated sodium percarbonate of example 1 has been incorporated in aliquid detergent formulation suitable for laundering/bleaching. Thedetergent liquid has the following composition:

-   Base (100 g): sodium salt of linear alkyl benzene sulfonate: 16% wt    -   C11 alcohol ethoxylate: 21% wt    -   Butoxy-propoxy-propanol: 19% wt    -   sodium citrate: 4% wt    -   [4-[N-nonanoyl-6-aminohexanoyloxy]benzene sulfonate]sodium salt:        6% wt    -   chloride salt of methyl quarternized polyethoxylated    -   hexamethylene diamine: 1.2% wt    -   ethylenediamine disuccinic acid: 1% wt    -   sodium carbonate: 7% wt    -   maleic-acrylic copolymer: 3% wt    -   protease: 0.40% wt    -   amylase: 0.8% wt    -   cellulase: 0.50% wt    -   suds suppressor: 1.5% wt    -   perfume: 0.5% wt    -   titanium dioxide: 0.5% wt    -   brightener: 0.14% wt    -   thixatrol ST: 0.1% wt    -   Speckles: 0.4% wt    -   miscellaneous: remainder    -   coated sodium percarbonate: 168 g

EXAMPLE 8 (ACCORDING TO THE INVENTION)

The coating is carried out in two phases. In the first step wetSodiumcarbonate peroxihydrate is taken from the plant after thecentrifuge. The moisture of the wet PCS is 9.1% H2O. 2-2.5 kg of thisproduct is filled in a LÖDIGE mixer M5 (ploughshare mixer, 5 litercapacity) and mixed at 250 (±50) RPM. The boric acid is addedcontinuously in the upper part of the mixer during 1-2 minutes followingthe starting of the apparatus. After adding the coating agent, themixing time is about 1 minute.

H3BO3 (solid) H3BO3 (% w/w in total mix) 2.5 H2O (% w/w in total mix)8.9 Temperature of mixer ambient

The used boric acid powder (Optibor SP, BORAX) has a particle sizedistribution of <3.0% on 250 μm sieve and a H3BO3 content of99.9-100.7%.

In a second step of the process, the wet, coated PCS coming from themixer is transferred in a lab fluid bed dryer (GLATT, WSG 1, Com. No.3198). The conditions of the drying step are:

H3BO3 (solid) Temperature of inlet air (° C.) 90-100 Temperature ofoutlet air (° C.) 85 Air flow (m³/h) 150 Drying time (min) 7-13

The warm product is removed from the dryer and cooled under ambiantconditions.

Results:

H3BO3 (solid) Available oxygen (%) 14.04 Coatinglevel (%) 2.5 Boron (%)0.43 LKB in base powder (μW/g) 4.3

The product shows at a coating level of 2.5% good results in the LKBvalue which is determined by a microcalorimeter LKB 2277 Bio ActivityMonitor. The available oxygen content is determined by permanganometrictitration, the boron content by ICP (SPECTRO, SPECTROFlame No. 2308/89).

EXAMPLE 9 (ACCORDING TO THE INVENTION)

The coating is carried out in two phases. In the first step drySodiumcarbonate per-oxihydrate is taken from the plant after the dryer.2-2.5 kg of this product is filled in a LÖDIGE mixer M5 (ploughsharemixer, 5 liter capacity) and mixed at 250 (±50) RPM. The borate/silicatesolution is added continuously in the upper part of the mixer during 1-2minutes following the starting of the apparatus. After adding thecoating agent, the mixing time is about 1 minute.

Borate/silicate solution Coating (% w/w in total mix) 2.8 H₂O (% w/w intotal mix) 3.4 Temperature of mixer ambient

The used borate silicate solution is composed of 129 g/kg Na₂O, 53 g/kgSiO₂, 43 g/kg B with a total solid content of 460 g/kg.

In a second step of the process, the wet, coated PCS coming from themixer is transferred in a lab fluid bed dryer (GLATT, WSG 1, Com. No.3198). The conditions of the drying step are:

Borate/silicate solution Temperature of inlet air (° C.) 90-100Temperature of outlet air (° C.) 85 Air flow (m³/h) 150 Drying time(min) 7-13

The warm product is removed from the dryer and cooled under ambiantconditions.

Results:

Borate/silicate solution Available oxygen (%) 13.96 Coatinglevel (%) 2.8Boron (%) 0.26 LKB in base powder (μW/g) 11

The LKB value is determined by a microcalorimeter LKB 2277 Bio ActivityMonitor. The available oxygen content is determined by permanganometrictitration, the boron content by ICP (SPECTRO, SPECTROFlame No. 2308/89).

EXAMPLE 10 (ACCORDING TO THE INVENTION)

The coating is carried out in two phases. In the first step drySodiumcarbonate peroxihydrate is taken from the plant after the dryer. 2kg of this product, 10% PCS-dust and the boric acid are filled in aLÖDIGE mixer M5 (ploughshare mixer, 5 liter capacity) and mixed at 250(±50) RPM. PCS-mother liquor is added continuously in the upper part ofthe mixer during 1-2 minutes following the starting of the apparatus.After adding the mother liquor, the mixing time is about 1 minute.

H₃BO₃ (solid) Coating (% w/w in total mix) 1.8 H₂O (% w/w in total mix)5.1 Temperature of mixer ambient

The used boric acid powder (Optibor SP, BORAX) has a particle sizedistribution of <3.0% on 250 μm sieve and a H₃BO₃ content of99.9-100.7%.

The analysis of the PCS-dust shows an available oxygen of 13.7%.

In a second step of the process, the wet, coated PCS coming from themixer is transferred in a lab fluid bed dryer (GLATT, WSG 1, Com. No.3198). The conditions of the drying step are:

H₃BO₃ (solid) Temperature of inlet air (° C.) 90–100 Temperature ofoutlet air (° C.) 85 Air flow (m³/h) 150 Drying time (min) 7–13

The warm product is removed from the dryer and cooled under ambiantconditions.

Results:

H₃BO₃ (solid) Available oxygen (%) 13.97 Coatinglevel (%) 1.8 Boron (%)0.32 LKB in base powder (μW/g) 3.4

The product shows at a coating level of 1.8% good results in the LKBvalue which is determined by a microcalorimeter LKB 2277 Bio ActivityMonitor. The available oxygen content is determined by permanganometrictitration, the boron content by ICP (SPECTRO, SPECTROFlame No. 2308/89).

EXAMPLE 11 (ACCORDING TO THE INVENTION)

The same operations as in example 1 have been carried out using aninitial uncoated sodium percarbonate having:

Avox = 14.7% by weight mean particle size = 634 μm bulk density = 990g/ml

The conditions of the first mixing step were:

H₃ BO₃ (% w/w in total mix) = 0.97 Na₃ PO₄ (% w/w in total mix) = 2.33water (% w/w in total mix) = 8 Temperature of mixer = Ambient

The second step has been carried out as in example 1. The coated PCSpresented an available oxygen content of 14.0% w/w and a mean particlesize of 905 μm.

1. A process for making coated alkali metal percarbonate particlesconsisting of: mixing an alkali metal percarbonate with at least oneparticulate coating agent composition in solid form to form a mixturewherein the coating agent composition comprises a coating agent selectedfrom the group consisting of orthoboric acid, metaboric acid, tetraboricacid, phosphates, sulfates and carbonates and is present in an amount ofless than 15% of the total weight of the mixture, wherein the amount ofwater present during mixing is at least 1.5% and no more than 8% byweight of the total weight of the mixture, and wherein said mixing stepis performed without carrying out, before mixing, a preliminarypreparation of a solution or a slurry of the coating agent.
 2. Theprocess according to claim 1, wherein the coating agent compositioncomprises at least one of orthoboric acid, metaboric acid, andtetraboric acid.
 3. The process according to claim 2, wherein thecoating agent composition comprises at least 96% by weight of one oforthoboric acid, metaboric acid, tetraboric acid and mixtures thereof.4. The process according to claim 1, wherein the amount of the coatingagent composition is at most 7% of the total weight of the mixture. 5.The process according to claim 2, wherein a coated alkali metalpercarbonate comprising from 0.1 to 0.5% by weight of boron is formed.6. The process according to claim 1, wherein the solid particulatecoating agent composition comprises a particulate coating agent having amean diameter of at most 500 μm.
 7. The process according to claim 1,wherein the alkali metal percarbonate comprises less than 1.5% by weightof water.
 8. A process for making coated alkali metal percarbonateparticles consisting of: mixing dry alkali metal percarbonate particlescomprising less than 1.5% by weight of water with at least oneparticulate coating agent composition in solid form in the presence ofan amount of water which does not exceed 8% of the total weight of themixture, wherein the coating agent composition comprises a coating agentselected from the group consisting of orthoboric acid, metaboric acid,tetraboric acid, phosphates, sulfates, and carbonates and is present inan amount of less than 15% of the total weight of the mixture, to formwet coated alkali metal percarbonate particles; drying the wet coatedalkali metal percarbonate particles to form dry coated alkali metalpercarbonate particles containing less than 1.5% by weight of water; andcollecting the dry coated alkali metal percarbonate particles, whereinthe amount of water present during mixing is at least 1.5% and no morethan 8% by weight of the total mixture, and wherein said mixing step isperformed without carrying out, before mixing, a preliminary preparationof a solution or a slurry of the coating agent.
 9. The process accordingto claim 8, wherein mixing is carried out by first mixing the dry alkalimetal percarbonate particles with the solid coating agent composition,and then adding water.
 10. The process according to claim 8, whereinmixing is carried out by mixing the dry alkali metal percarbonateparticles simultaneously with water and the coating agent composition.11. The process according to claim 1, wherein the alkali metalpercarbonate is in the form of wet particles.
 12. A process for makingcoated alkali metal percarbonate particles consisting of: mixing a wetcake of alkali metal percarbonate particles with at least oneparticulate coating agent composition in solid form and optionally withdry alkali metal percarbonate particles, to form a wet cake of coatedalkali metal percarbonate particles wherein the coating agentcomposition comprises a coating agent selected from the group consistingof orthoboric acid, metaboric acid, tetraboric acid, phosphates,sulfates, and carbonates and is present in an amount of less than 15% ofthe total weight of the mixture; drying the wet cake of coated alkalimetal percarbonate particles to form dry coated alkali metalpercarbonate particles; and collecting the dry coated alkali metalpercarbonate particles, wherein the amount of water present duringmixing is at least 1.5% and no more than 8% by weight of the totalmixture, and wherein said mixing step is performed without carrying out,before mixing, a preliminary preparation of a solution or a slurry ofthe coating agent.
 13. The process according to claim 1, wherein thealkali metal percarbonate is mixed with the solid particulate coatingagent in the presence of an aqueous liquid.
 14. The process according toclaim 3, wherein the coating agent composition comprises 100% by weightof one of orthoboric acid, metaboric acid, tetraboric acid and mixturesthereof.
 15. The process according to claim 2, wherein a coated alkalimetal percarbonate comprising from 0.15% to 0.3% by weight of boron isformed.
 16. The process according to claim 1, wherein the solidparticulate coating agent has a mean diameter of at most 250 μm.
 17. Theprocess according to claim 1, wherein the alkali metal percarbonate isin the form of dry particles obtained by fluid bed granulation.
 18. Theprocess according to claim 1, wherein the coating agent is orthoboricacid.
 19. The process according to claim 1, wherein the coating agent ismetaboric acid.
 20. The process according to claim 1, wherein thecoating agent is tetraboric acid.
 21. A process for making coated alkalimetal percarbonate particles consisting of: mixing an alkali metalpercarbonate with a dry composition consisting of a single particulatecoating agent in solid form to form a mixture without carrying out,before mixing, a preliminary preparation of a solution or a slurry ofthe coating agent wherein the coating agent is present in an amount ofless than 15% of the total weight of the mixture, wherein the amount ofwater present during mixing is at least 1.5% and no more than 8% byweight of the total weight of the mixture.
 22. The process according toclaim 21, wherein the coating agent is orthoboric acid.
 23. The processaccording to claim 21, wherein the coating agent is metaboric acid. 24.The process according to claim 21, wherein the coating agent istetraboric acid.
 25. The process according to claim 1, wherein theamount of water present during mixing is at least 1.5% and no more than5.1% by weight of the total weight of the mixture.
 26. The processaccording to claim 8, wherein the dry alkali metal percarbonateparticles are mixed with the particulate coating agent composition inthe presence of an amount of water which does not exceed 5.1% of thetotal weight of the mixture, to form wet coated alkali metalpercarbonate particles.
 27. The process according to claim 21, whereinthe amount of water present during mixing is at least 1.5% and no morethan 5.1% by weight of the total weight of the mixture.
 28. A processfor making coated alkali metal percarbonate particles consisting of:mixing an alkali metal percarbonate with a particulate coating agentcomposition in solid form to form a mixture wherein the coating agent ispresent in an amount of less than 15% of the total weight of themixture, wherein the amount of water present during mixing is at least1.5% and no more than 8% by weight of the total weight of the mixture,wherein said mixing step is performed without carrying out, beforemixing, a preliminary preparation of a solution or a slurry of thecoating agent, and wherein the coating agent composition consists ofboric acid.